Because for the most part, historical wars have not been heavily focused on tank combat. In the few occasions where there were major tank action they also didn't last long enough to fill out a whole narrative over a few months. Historical-themed maps can work for sure (but limiting vehicles you can play has its own issues too), but this won't work for a full length BP.
I don't know about you, but I'm pretty sure there weren't any T-72s in Vietnam. Reenacting historical battles with non-historical vehicles feels weird to me.
One thing I do want to see is the contract system featuring more lore elements, so the missions don't feel like they exist solely for gameplay.
Every BP we've had so far did not feel like there were a lot of lore involved, no doubt about that. I think all the lore elements got removed from BPs after SH (the Asian BP), so we ended up with grindfests that had nothing to do with the lore itself. It would be nice if future BPs return to actually having lore involved so they don't feel like cheap cashgrabs.
One thing I do want changed for BPs is their investment cost. Currently they are strictly pay2grind or paymore2grindless, it would be nice if BPs can be separated into free and paid tiers so that players who are only interested in the lore can still participate, and if they want more rewards such as high tier premium vehicles and the coin shop they will need to pay for access.